Spunbonded polyolefin sheet materials are used in a variety of applications, including protective vehicle covers, chemical and biological protective clothing, and industrial cleanroom coveralls. In the aforementioned applications, panels of spunbonded polyolefin sheet material must be joined. Such panels are conventionally joined by sewing, heat welding, adhesive bonding, or ultrasonic welding. In certain applications, the seams that join the panels of sheet material together must be nearly as strong, as moisture and particulate impermeable, and as flexible as the sheet material itself.
A polyolefin sheet material that is used in many protective garments is TYVEK.RTM. spunbonded polyolefin sheet sold by E. I. du Pont de Nemours and Company of Wilmington, Del. (hereinafter "DuPont". TYVEK.RTM. is a registered trademark of DuPont. TYVEK.RTM. sheets are made from flashspun polyethylene plexifilamentary fibers that have been thermally bonded as disclosed in U.S. Pat. No. 3,169,899 issued to Steuber. TYVEK.RTM. (sheet material is especially useful in protective garment applications because it is both strong and lightweight, and because it is substantially impermeable to particulates and liquids.
In making protective garments from panels of spunbonded polyolefin sheet material, it has proved difficult to join the panels in a manner that does not compromise the effectiveness of the garment. When garments are sewn from panels of spunbonded polyolefin, needle holes make the garments too porous for many protective applications. Likewise, the needle holes that result from the stitching of seams can emit microscopic fiber particles that make garments sewn from spunbonded polyolefin unacceptable for industrial cleanroom applications. In addition, exposed seam edges and seam indentations tend to collect dust and other particulates that are undesirable for cleanroom applications or for hazardous dust protection applications. Unfortunately, it is very difficult to heat bond spunbonded polyolefin panels because the heat and pressure of a conventional heat bonding process reduces fiber orientation, which weakens the sheet material and makes the sheet material hard and brittle.
U.S. Pat. No. 4,938,817 discloses a method for ultrasonically bonding panels of TYVEK.RTM. spunbonded polyolefin directly to each other in the manner shown in FIGS. 1 and 2. In order to make a sufficiently strong seam according to the process of U.S. Pat. No. 4,938,817, two spunbonded polyolefin panels 10 and 12 are first placed on top of each other and lined up along a common edge 14, as shown in FIG. 1. Panels 10 and 12 are next ultrasonically bonded along a strip 16 near the edge 14 using a bonding pattern of short dashed lines 18. In order to make the seam sufficiently strong, sheet 10 must be subsequently folded over on itself, as shown in FIG. 2, and ultrasonically bonded against the previously bonded strip 16 using the same dashed line bonding pattern.
Although the seam formed as shown in FIGS. 1 and 2 is strong, the requirement that each seam be ultrasonically bonded two times makes seaming protective garments by this method time consuming and expensive. Accordingly, there is a need for an alternative method for effectively joining panels of spunbonded polyolefin sheet material that does not require bonding each seam two times to achieve sufficient strength, but also does not leave exposed panel edges.